The present invention relates to a method for reducing the content of spurious signals, including the image frequency, in the output signal of a mixer comprising at least two modulators the first inputs which are supplied with a carrier signal of the frequency f.sub.o while their second inputs are supplied with a modulation signal of the frequency f.sub.m in such a manner that the carrier and modulation signals supplied to the one of the at least two modulators are phase-shifted in a predetermined manner relative to the carrier or modulation signal supplied to the at least one other modulator and whose output signals are combined to the output signal of the mixer which has a frequency equal to the sum of the frequencies of the carrier signal and the modulation signal. Mixers of this type have been generally known in the form of quadrature mixtures containing two modulators to which the modulation signal is supplied, phase-shifted by 90.degree.. These quadrature mixers are in extensive use because, theoretically, all other mixture products, including the image frequency, are suppressed, except for the desired output signal having the sum frequency f.sub.o +f.sub.m or f.sub.o -f.sub.m. In reality, however, no pure output signal of the frequency f.sub.o +f.sub.m or f.sub.o -f.sub.m can be achieved, not even when the ring modulators are tuned very carefully; instead, there also appear output signals having the frequencies of the other mixture products. The reasons for this are substantially seen in the fact that the characteristic curves of the modulators are not purely quadratic, that no complete symmetry of the modulators can be achieved, that the 90.degree. phase shifters used in the modulator are responsive to frequency and load, that cross-talking occurs in the circuitry, and that finally the modulators do not have constant input and output impedances. Consequently, it has been possible with the aid of such quadrature mixers only to suppress the undesirable mixture products up to approx. 30 db relative to the output signal having the sum frequency. In addition, complex compensation circuits are used for suppressing any spurious signals, such as the compensation circuits known from DE 35 16 492 A1 which use four modulators.
On the other hand, quadrature mixers have quite considerable importance, and this not only as modulators or demodulators, but alos as rf generators of high stability. For generating the rf carrier signal, an oscillator of high stability may be used, while for generating the mixed signal a generator will do whose stability is lower than the stability of the oscillator generating the carrier signal, by the same factor by which the frequency of the modulation signal is lower than the frequency of the carrier signal. It is, however, important, especially in those applications for which such highly stable generators are required, to obtain also a highly pure output signal so that it would be desirable to improve the purity of the output signal of such a quadrature mixer without increasing the circuitry input by the use of additional components, such as modulators and adder or subtractor stages, and this the more as it is a fact that every component contributes itself, to a certain degree, towards distorting the signal and is accordingly detrimental to the generation of an output signal of very high purity.